This invention relates generally to an array of photo diode sensors. In particular, it relates to an array of photo diode sensors which include a ring of guard diodes around the periphery of the array.
An array of image sensors or light sensitive sensors detect the intensity of light received by the image sensors. The image sensors typically generate electronic signals that have amplitudes that are proportionate to the intensity of the light received by the image sensors. The image sensors convert an optical image into a set of electronic signals. The electronic signals may represent intensities of colors of light received by the image sensors. The electronic signals can be conditioned and sampled to allow image processing.
Integration of the image sensors with signal processing circuitry is becoming more important because integration enables miniaturization and simplification of imaging systems. Integration of image sensors along with analog and digital signal processing circuitry allows electronic imaging systems to be low cost, compact and require low power.
Historically, image sensors have predominantly been charged coupled devices (CCDs). CCDs are relatively small and can provide a high-fill factor. However; CCDs are very difficult to integrate with digital and analog circuitry. Further, CCDs dissipate large amounts of power and suffer from image smearing problems.
An alternative to CCD sensors are active pixel sensors. Active pixel sensors can be fabricated using standard CMOS processes. Therefore, active pixel sensors can easily be integrated with digital and analog signal processing circuitry. Further, CMOS circuits dissipate small amounts of power.
FIG. 1 shows a cross-section of an array of photo diode sensors. The array is formed on a substrate 10. The array includes photo diode sensors 12, 14, 16. Cathodes of the photo diode sensors 12, 14, 16 are electrically connected to the substrate 10. Anodes of the photo diode sensors 12, 14, 16 are electrically connected to a transparent conductive layer 18. The transparent conductive layer 18 is electrically connected to a bias voltage which ensures that the photo diode sensors 12, 14, 16 are reverse biased.
FIG. 2 is a schematic of a typical circuit included on the substrate 10 which is electrically connected to each photo diode sensor 12, 14, 16. The circuit includes a switch 22 which drives the cathode of each photo diode sensor 12, 14, 16 to an initial cathode voltage and charges a cathode capacitor 24. The switch 22 is subsequently opened and the cathode capacitor 24 discharges as the photo diode sensor connected to the cathode capacitor 24 conducts charge. The rate in which the cathode capacitor 24 discharges is dependent upon the intensity of light received by the photo diode sensor connected to the cathode capacitor 24. Therefore, the intensity of light received by the photo diode sensor can be determined by sampling the voltage on the cathode capacitor 24 a period of time after the switch 22 has been opened.
The charge conducted by a reverse biased diode is generated in the space charge region and neutral region of the diode. Photodiode sensors are configured so that the charge conducted by a reverse biased photo diode sensor is generated in the space charged region. The magnitude of the charge conducted is directly dependent on the volume of the space charged region. In an array of photo diode sensors, the space charged region of each photo diode sensor extends beyond the physical boundaries of the photo diode sensor. The space charged region is defined by the electric field between the anode and the cathode of the photo diode sensor.
FIG. 1 includes arrows within each photo diode sensor 12, 14, 16 which depict the electric field between the anode and the cathode of each photo diode sensor 12, 14, 16. Each of the photo diode sensor 12, 14, 16 are biased the same way. That is, the anodes and the cathodes of all the photo diode sensor 12, 14, 16 within the array are biased with approximately the same voltage potentials.
The photo diode sensors 14, 16 which are within the interior of the array of photo diode sensors, include electric fields which are approximately symmetrical around the photo diode sensor. Therefore, the interior photo diode sensors 14, 16 conduct approximately the same amount of charge when exposed to uniform illumination or light. However, the photo diode sensor 12 at the edge of the array is not surrounded by a symmetrical electric field. The amount of charge conducted by the edge photo diode sensor 12 when exposed to the same uniform illumination is greater than the charge conducted by the interior photo diode sensors 14, 16 because of the non-symmetric electric field surrounding the edge photo diode sensor 12.
All edge diodes of an array of photo diode sensors, like the edge diode 12, include non-symmetric electric fields. Therefore, the edge diodes of an array of photo diode sensors do not conduct the same amount of charge as interior photo diode sensors of the array when exposed to uniform illumination. Additionally, the non-symmetric electric fields of the edge diodes can influence of the electric fields of the interior photo diode sensors.
It is desirable to have an array of photo diode sensors in which photo diode sensors at the edge of the array include symmetric electric fields, and conduct the same amount of charge as interior photo diode sensors when exposed to uniform light. It is desirable that diodes at the edge of the array of photo diodes have electric fields that do not influence the electric fields of the interior photo diode sensors. Additionally, it is desirable that the array of photo diode sensors be manufacturable without adding extra processing and fabrication steps.
The present invention is an image sensor array. The image sensor array includes photo diode sensors in which photo diode sensors at the edge of the array have symmetric electric fields, and conduct the same amount of charge as interior photo diode sensors when exposed to uniform light. The array of photo diode sensors are manufacturable without adding extra processing and fabrication steps.
A first embodiment of this invention includes an image sensor array. The image sensor array includes a substrate. An array of photo sense diodes are electrically interconnected to the substrate. The photo sense diodes conduct charge at a rate proportional to the intensity of light received by the photo sense diodes. A ring of guard diodes is located around the periphery of the array of photo sense diodes. Each guard diode has a guard diode anode connected to a predetermined anode voltage and a guard diode cathode connected to a static guard cathode voltage.
A second embodiment is similar to the first embodiment. The second embodiment further includes the static guard cathode voltage being a predetermined constant voltage. The predetermined constant voltage is selected so that a guard voltage potential across each guard diode is equal to an average of sense voltages between anodes and cathodes of the photo sense diodes.
A third embodiment is similar to the first embodiment. The third embodiment includes the photo sense diodes and the guard diodes being PIN diodes.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.